Caption: Tissue macrophages, T lymphocytes (2) and human red blood cells from a leg wound, coloured scanning electron micrograph (SEM). One monocyte (orange) that has migrated into the wound area, is still developing into a tissue macrophage. A tissue macrophage is a large, mature phagocyte that can ingest and destroy invading microbes, foreign particles, cancerous or diseased cells and cellular debris. A monocyte is a circulating phagocyte that ingests microbes, invading particles, and cellular debris. Monocytes leave the blood stream and usually mature into tissue macrophages. Monocytes and macrophages are both part of the reticuloendothelial system. Pre-T cells circulate in the blood before migrating to the thymus where they develop into specialized cells (helper T cells and killer T cells) that are able to identify antigens and infected tissue cells. Magnification: x800 when shortest axis printed at 25 millimetres.